Whatever you do don't wad up napkins and put em in the other back pocket to even you out.

We would have to call you George.

Seriously though, back pocket would probably be my last option. The back pocket of my jeans/slacks are usually loose, so I would be worried about it working it's way up when moving around sitting down. I also trust my front pockets against pick pockets more then my back pockets. Haven't ever encountered a pick pocket but there's always a first.